Vibration isolation systems which include a coil spring, mostly in form of a steel spring, are known. In such a system a load to be isolated from the ground is mounted on three or more vibration isolators which include a coil spring that is effective at least in a vertical direction.
Such coil springs provide for passive vibration isolation in a very simple way, in particular even for quite heavy components.
Above the natural frequency of the spring-mass system, vibration isolation is achieved.
The isolation effect of such vibration isolation systems can be improved by configuring the vibration isolation system as an active vibration isolation system, in particular when used to mount sensitive equipment in semiconductor industry, such as lithography apparatus.
In such an active vibration isolation system, sensors are arranged on the load to be isolated and/or on the ground, and via a control loop at least one actuator is controlled, which actively counteracts vibrations. Especially non-contact force actuators are used as the actuator, in particular based on the Lorentz principle.
The employed mechanical spring systems are adapted to the weight of the load to be mounted. This is necessary because in the region of load limit a mechanical spring does no longer behave linearly and can be damaged.
Since the deflection of a coil spring is substantially linearly related to the applied force, it will be understood that the level of the anti-vibration mounted load varies with the weight thereof.
In order to compensate for variations of the load supported with isolation, or for a non-uniform force distribution, it is known in practice to provide for height adjustment in such a mechanical spring system including a coil spring.
Such a level correction may for example be implemented in a very simple manner using a height-adjustable spring plate.
An operator adjusts the vibration isolation system in a resting position.
Adjustment is for example effected by means of an adjusting element in form of a nut which is guided on a threaded rod.
However, it has now been found that during mechanical height adjustment torques may be generated in the mechanical spring system, which may result in a horizontal force. This horizontal force component usually leads to a horizontal shift of the anti-vibration mounted load, which is undesirable.
In active vibration isolation systems, a horizontal shift is particularly disadvantageous, since the opposite components of non-contact actuators will be offset thereby relative to one another. In case of a Lorentz coil, for example, this may result in that the plunger coil touches the permanent magnet, or it will at least result in that the size of the air gap between the plunger coil and the permanent magnet is no longer in the desired range.
Thus, after height adjustment the vibration isolation system additionally has to be adjusted in the horizontal direction, which is cumbersome.